Storing a point in time coherently for a distributed storage system

ABSTRACT

A plurality of computing devices are communicatively coupled to each other via a network, and each of the plurality of computing devices is operably coupled to one or more of a plurality of storage devices. The computing devices may take snapshots to store points in time coherently for a distributed storage system.

PRIORITY CLAIM

This application claims priority to the following application, which ishereby incorporated herein by reference:

U.S. provisional patent application 62/699,902 titled “STORING A POINTIN TIME COHERENTLY FOR A DISTRIBUTED STORAGE SYSTEM” filed on Jul. 18,2018.

BACKGROUND

Limitations and disadvantages of conventional approaches to data storagewill become apparent to one of skill in the art, through comparison ofsuch approaches with some aspects of the present method and system setforth in the remainder of this disclosure with reference to thedrawings.

INCORPORATION BY REFERENCE

U.S. patent application Ser. No. 15/243,519 titled “Distributed ErasureCoded Virtual File System” is hereby incorporated herein by reference inits entirety.

BRIEF SUMMARY

Methods and systems are provided for storing a point in time coherentlyfor files in a distributed storage system substantially as illustratedby and/or described in connection with at least one of the figures, asset forth more completely in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates various example configurations of a virtual filesystem in accordance with aspects of this disclosure.

FIG. 2 illustrates an example configuration of a virtual file systemnode in accordance with aspects of this disclosure.

FIG. 3 illustrates an example configuration of a virtual file systemnode in accordance with aspects of this disclosure.

FIG. 4 illustrates a user interface for a storage system in accordancewith an example implementation of this disclosure.

FIG. 5A illustrates an example flow diagram of taking a snapshot inaccordance with an example implementation of this disclosure.

FIG. 5B illustrates an example flow diagram of changes to a file systemin accordance with an example implementation of this disclosure.

FIG. 6 illustrates an example flow diagram of deletion of a snapshot inaccordance with an example implementation of this disclosure.

FIG. 7 illustrates an example flow diagram of deletion of a snapshot inaccordance with an example implementation of this disclosure.

DETAILED DESCRIPTION

Traditionally, file systems use a centralized control over the metadatastructure (e.g., directories, files, attributes, file contents). If alocal file system is accessible from a single server and that serverfails, the file system's data may be lost if as there is no furtherprotection. To add protection, some file systems (e.g., as provided byNetApp) have used one or more pairs of controllers in an active-passivemanner to replicate the metadata across two or more computers. Othersolutions have used multiple metadata servers in a clustered way (e.g.,as provided by IBM GPFS, Dell EMC Isilon, Lustre, etc.). However,because the number of metadata servers in a traditional clustered systemis limited to small numbers, such systems are unable to scale.

The systems in this disclosure are applicable to small clusters and canalso scale to many, many thousands of nodes. An example embodiment isdiscussed regarding non-volatile memory (NVM), for example, flash memorythat comes in the form of a solid-state drive (SSD). The NVM may bedivided into 4 kB “blocks” and 128 MB “chunks.” “Extents” may be storedin volatile memory such as, for example, RAM, for fast access, and maybe backed up by NVM storage as well. An extent may store pointers forblocks, e.g., 256 pointers to 1 MB of data stored in blocks. In otherembodiments, larger or smaller memory divisions may also be used.Metadata functionality in this disclosure may be effectively spreadacross many servers. For example, in cases of “hot spots” where a largeload is targeted at a specific portion of the file system's namespace,this load can be distributed across a plurality of nodes.

FIG. 1 illustrates various example configurations of a virtual filesystem (VFS) in accordance with aspects of this disclosure. Shown inFIG. 1 is a local area network (LAN) 102 comprising one or more VFSnodes 120 (indexed by integers from 1 to J, for j≥1), and optionallycomprising (indicated by dashed lines): one or more dedicated storagenodes 106 (indexed by integers from 1 to M, for M≥1), one or morecompute nodes 104 (indexed by integers from 1 to N, for N≥1), and/or anedge router that connects the LAN 102 to a remote network 118. Theremote network 118 optionally comprises one or more storage services 114(indexed by integers from 1 to K, for K≥1), and/or one or more dedicatedstorage nodes 115 (indexed by integers from 1 to L, for L≥1).

Each VFS node 120 j (j an integer, where 1≤j≤J) is a networked computingdevice (e.g., a server, personal computer, or the like) that comprisescircuitry for running VFS processes and, optionally, client processes(either directly on an operating system of the device 104 n and/or inone or more virtual machines running in the device 104 n).

The compute nodes 104 are networked devices that may run a VFS frontendwithout a VFS backend. A compute node 104 may run VFS frontend by takingan SR-IOV into the NIC and consuming a complete processor core.Alternatively, the compute node 104 may run the VFS frontend by routingthe networking through a Linux kernel networking stack and using kernelprocess scheduling, thus not having the requirement of a full core. Thisis useful if a user does not want to allocate a complete core for theVFS or if the networking hardware is incompatible with the VFSrequirements.

FIG. 2 illustrates an example configuration of a VFS node in accordancewith aspects of this disclosure. A VFS node comprises a VFS frontend 202and driver 208, a VFS memory controller 204, a VFS backend 206, and aVFS SSD agent 214. As used in this disclosure, a “VFS process” is aprocess that implements one or more of: the VFS frontend 202, the VFSmemory controller 204, the VFS backend 206, and the VFS SSD agent 214.Thus, in an example implementation, resources (e.g., processing andmemory resources) of the VFS node may be shared among client processesand VFS processes. The processes of the VFS may be configured to demandrelatively small amounts of the resources to minimize the impact on theperformance of the client applications. The VFS frontend 202, the VFSmemory controller 204, and/or the VFS backend 206 and/or the VFS SSDagent 214 may run on a processor of the host 201 or on a processor ofthe network adaptor 218. For a multi-core processor, different VFSprocess may run on different cores, and may run a different subset ofthe services. From the perspective of the client process(es) 212, theinterface with the virtual file system is independent of the particularphysical machine(s) on which the VFS process(es) are running. Clientprocesses only require driver 208 and frontend 202 to be present inorder to serve them.

The VFS node may be implemented as a single tenant server (e.g.,bare-metal) running directly on an operating system or as a virtualmachine (VM) and/or container (e.g., a Linux container (LXC)) within abare-metal server. The VFS may run within an LXC container as a VMenvironment. Thus, inside the VM, the only thing that may run is the LXCcontainer comprising the VFS. In a classic bare-metal environment, thereare user-space applications and the VFS runs in an LXC container. If theserver is running other containerized applications, the VFS may runinside an LXC container that is outside the management scope of thecontainer deployment environment (e.g. Docker).

The VFS node may be serviced by an operating system and/or a virtualmachine monitor (VMM) (e.g., a hypervisor). The VMM may be used tocreate and run the VFS node on a host 201. Multiple cores may resideinside the single LXC container running the VFS, and the VFS may run ona single host 201 using a single Linux kernel. Therefore, a single host201 may comprise multiple VFS frontends 202, multiple VFS memorycontrollers 204, multiple VFS backends 206, and/or one or more VFSdrivers 208. A VFS driver 208 may run in kernel space outside the scopeof the LXC container.

A single root input/output virtualization (SR-IOV) PCIe virtual functionmay be used to run the networking stack 210 in user space 222. SR-IOVallows the isolation of PCI Express, such that a single physical PCIExpress can be shared on a virtual environment and different virtualfunctions may be offered to different virtual components on a singlephysical server machine. The I/O stack 210 enables the VFS node tobypasses the standard TCP/IP stack 220 and communicate directly with thenetwork adapter 218. A Portable Operating System Interface for uniX(POSIX) VFS functionality may be provided through lockless queues to theVFS driver 208. SR-IOV or full PCIe physical function address may alsobe used to run non-volatile memory express (NVMe) driver 214 in userspace 222, thus bypassing the Linux IO stack completely. NVMe may beused to access non-volatile storage device 216 attached via a PCIExpress (PCIe) bus. The non-volatile storage device 216 may be, forexample, flash memory that comes in the form of a solid-state drive(SSD) or Storage Class Memory (SCM) that may come in the form of an SSDor a memory module (DIMM). Other example may include storage classmemory technologies such as 3D-XPoint.

The SSD may be implemented as a networked device by coupling thephysical SSD, for example, the non-volatile storage device 216, with theSSD agent 214 and networking 210. Alternatively, the SSD may beimplemented as a network-attached NVMe SSD 242 or 244 by using a networkprotocol such as NVMe-oF (NVMe over Fabrics). NVMe-oF may allow accessto the NVMe device using redundant network links, thereby providing ahigher level or resiliency. Network adapters 226, 228, 230 and 232 maycomprise hardware acceleration for connection to the NVMe SSD 242 and244 to transform them into networked NVMe-oF devices without the use ofa server. The NVMe SSDs 242 and 244 may each comprise two physicalports, and all the data may be accessed through either of these ports.

Each client process/application 212 may run directly on an operatingsystem or may run in a virtual machine and/or container serviced by theoperating system and/or hypervisor. A client process 212 may read datafrom storage and/or write data to storage in the course of performingits primary function. The primary function of a client process 212,however, is not storage-related (i.e., the process is only concernedthat its data is reliably stored and is retrievable when needed, and notconcerned with where, when, or how the data is stored). Exampleapplications which give rise to such processes include: email servers,web servers, office productivity applications, customer relationshipmanagement (CRM), animated video rendering, genomics calculation, chipdesign, software builds, and enterprise resource planning (ERP).

A client application 212 may make a system call to the kernel 224 whichcommunicates with the VFS driver 208. The VFS driver 208 puts acorresponding request on a queue of the VFS frontend 202. If several VFSfrontends exist, the driver may load balance accesses to the differentfrontends, making sure a single file/directory is always accessed viathe same frontend. This may be done by “sharding” the frontend based onthe ID of the file or directory. The VFS frontend 202 provides aninterface for routing file system requests to an appropriate VFS backendbased on the bucket that is responsible for that operation. Theappropriate VFS backend may be on the same host or it may be on anotherhost.

The VFS backend 206 hosts several buckets, each one of them services thefile system requests that it receives and carries out tasks to otherwisemanage the virtual file system (e.g., load balancing, journaling,maintaining metadata, caching, moving of data between tiers, removingstale data, correcting corrupted data, etc.)

The VFS SSD agent 214 handles interactions with a respectivenon-volatile storage device 216. This may include, for example,translating addresses, and generating the commands that are issued tothe storage device (e.g., on a SATA, SAS, PCIe, or other suitable bus).Thus, the VFS SSD agent 214 operates as an intermediary between anon-volatile storage device 216 and the VFS backend 206 of the virtualfile system. The SSD agent 214 could also communicate with a standardnetwork storage device supporting a standard protocol such as NVMe-oF(NVMe over Fabrics).

FIG. 3 illustrates another representation of a virtual file system inaccordance with an example implementation of this disclosure. In FIG. 3,the element 302 represents memory resources (e.g., DRAM and/or othershort-term memory) and processing (e.g., x86 processor(s), ARMprocessor(s), NICs, ASICs, FPGAs, and/or the like) resources of variousnode(s) (compute, storage, and/or VFS) on which resides a virtual filesystem, such as described regarding FIG. 2A above. The element 308represents the one or more physical non-volatile storage devices 216which provide the long term storage of the virtual file system.

As shown in FIG. 3, the physical storage is organized into a pluralityof distributed failure resilient address spaces (DFRASs) 318. Each ofwhich comprises a plurality of chunks 310, which in turn comprises aplurality of blocks 312. The organization of blocks 312 into chunks 310is only a convenience in some implementations and may not be done in allimplementations. Each block 312 stores committed data 316 (which maytake on various states, discussed below) and/or metadata 314 thatdescribes or references committed data 316.

The organization of the storage 308 into a plurality of DFRASs enableshigh performance parallel commits from many—perhaps all—of the nodes ofthe virtual file system (e.g., all nodes 104 ₁-104 _(N), 106 ₁-106 _(M),and 120 ₁-120 _(J) of FIG. 1 may perform concurrent commits inparallel). In an example implementation, each of the nodes of thevirtual file system may own a respective one or more of the plurality ofDFRAS and have exclusive read/commit access to the DFRASs that it owns.

Each bucket owns a DFRAS, and thus does not need to coordinate with anyother node when writing to it. Each bucket may build stripes across manydifferent chunks on many different SSDs, thus each bucket with its DFRAScan choose what “chunk stripe” to write to currently based on manyparameters, and there is no coordination required in order to do so oncethe chunks are allocated to that bucket. All buckets can effectivelywrite to all SSDs without any need to coordinate.

Each DFRAS being owned and accessible by only its owner bucket that runson a specific node allows each of the nodes of the VFS to control aportion of the storage 308 without having to coordinate with any othernodes (except during [re]assignment of the buckets holding the DFRASsduring initialization or after a node failure, for example, which may beperformed asynchronously to actual reads/commits to storage 308). Thus,in such an implementation, each node may read/commit to its buckets'DFRASs independently of what the other nodes are doing, with norequirement to reach any consensus when reading and committing tostorage 308. Furthermore, in the event of a failure of a particularnode, the fact the particular node owns a plurality of buckets permitsmore intelligent and efficient redistribution of its workload to othernodes (rather the whole workload having to be assigned to a single node,which may create a “hot spot”). In this regard, in some implementationsthe number of buckets may be large relative to the number of nodes inthe system such that any one bucket may be a relatively small load toplace on another node. This permits fine grained redistribution of theload of a failed node according to the capabilities and capacity of theother nodes (e.g., nodes with more capabilities and capacity may begiven a higher percentage of the failed nodes buckets).

To permit such operation, metadata may be maintained that maps eachbucket to its current owning node such that reads and commits to storage308 can be redirected to the appropriate node.

Load distribution is possible because the entire file system metadataspace (e.g., directory, file attributes, content range in the file,etc.) can be broken (e.g., chopped or sharded) into small, uniformpieces (e.g., “shards”). For example, a large system with 30 k serverscould chop the metadata space into 128 k or 256 k shards.

Each such metadata shard may be maintained in a “bucket.” Each VFS nodemay have responsibility over several buckets. When a bucket is servingmetadata shards on a given backend, the bucket is considered “active” orthe “leader” of that bucket. Typically, there are many more buckets thanVFS nodes. For example, a small system with 6 nodes could have 120buckets, and a larger system with 1,000 nodes could have 8 k buckets.

Each bucket may be active on a small set of nodes, typically 5 nodesthat that form a penta-group for that bucket. The cluster configurationkeeps all participating nodes up-to-date regarding the penta-groupassignment for each bucket.

Each penta-group monitors itself. For example, if the cluster has 10 kservers, and each server has 6 buckets, each server will only need totalk with 30 different servers to maintain the status of its buckets (6buckets will have 6 penta-groups, so 6*5=30). This is a much smallernumber than if a centralized entity had to monitor all nodes and keep acluster-wide state. The use of penta-groups allows performance to scalewith bigger clusters, as nodes do not perform more work when the clustersize increases. This could pose a disadvantage that in a “dumb” mode asmall cluster could actually generate more communication than there arephysical nodes, but this disadvantage is overcome by sending just asingle heartbeat between two servers with all the buckets they share (asthe cluster grows this will change to just one bucket, but if you have asmall 5 server cluster then it will just include all the buckets in allmessages and each server will just talk with the other 4). Thepenta-groups may decide (i.e., reach consensus) using an algorithm thatresembles the Raft consensus algorithm.

Each bucket may have a group of compute nodes that can run it. Forexample, five VFS nodes can run one bucket. However, only one of thenodes in the group is the controller/leader at any given moment.Further, no two buckets share the same group, for large enough clusters.If there are only 5 or 6 nodes in the cluster, most buckets may sharebackends. In a reasonably large cluster there many distinct node groups.For example, with 26 nodes, there are more than

$\text{64,000}\left( \frac{26!}{{5!}*{\left( {26 - 5} \right)!}} \right)$

possible five-node groups (i.e., penta-groups).

All nodes in a group know and agree (i.e., reach consensus) on whichnode is the actual active controller (i.e., leader) of that bucket. Anode accessing the bucket may remember (“cache”) the last node that wasthe leader for that bucket out of the (e.g., five) members of a group.If it accesses the bucket leader, the bucket leader performs therequested operation. If it accesses a node that is not the currentleader, that node indicates the leader to “redirect” the access. Ifthere is a timeout accessing the cached leader node, the contacting nodemay try a different node of the same penta-group. All the nodes in thecluster share common “configuration” of the cluster, which allows thenodes to know which server may run each bucket.

Each bucket may have a load/usage value that indicates how heavily thebucket is being used by applications running on the file system. Forexample, a server node with 11 lightly used buckets may receive anotherbucket of metadata to run before a server with 9 heavily used buckets,even though there will be an imbalance in the number of buckets used.Load value may be determined according to average response latencies,number of concurrently run operations, memory consumed or other metrics.

Redistribution may also occur even when a VFS node does not fail. If thesystem identifies that one node is busier than the others based on thetracked load metrics, the system can move (i.e., “fail over”) one of itsbuckets to another server that is less busy. However, before actuallyrelocating a bucket to a different host, load balancing may be achievedby diverting writes and reads. Because each write may end up on adifferent group of nodes, decided by the DFRAS, a node with a higherload may not be selected to be in a stripe to which data is beingwritten. The system may also opt to not serve reads from a highly loadednode. For example, a “degraded mode read” may be performed, wherein ablock in the highly loaded node is reconstructed from the other blocksof the same stripe. A degraded mode read is a read that is performed viathe rest of the nodes in the same stripe, and the data is reconstructedvia the failure protection. A degraded mode read may be performed whenthe read latency is too high, as the initiator of the read may assumethat that node is down. If the load is high enough to create higher readlatencies, the cluster may revert to reading that data from the othernodes and reconstructing the needed data using the degraded mode read.

Each bucket manages its own distributed erasure coding instance (i.e.,DFRAS 318) and does not need to cooperate with other buckets to performread or write operations. There are potentially thousands of concurrent,distributed erasure coding instances working concurrently, each for thedifferent bucket. This is an integral part of scaling performance, as iteffectively allows any large file system to be divided into independentpieces that do not need to be coordinated, thus providing highperformance regardless of the scale.

Each bucket handles all the file systems operations that fall into itsshard. For example, the directory structure, file attributes, and filedata ranges will fall into a particular bucket's jurisdiction.

An operation done from any frontend starts by finding out what bucketowns that operation. Then the backend leader, and the node, for thatbucket is determined. This determination may be performed by trying thelast-known leader. If the last-known leader is not the current leader,that node may know which node is the current leader. If the last-knownleader is not part of the bucket's penta-group anymore, that backendwill let the front end know that it should go back to the configurationto find a member of the bucket's penta-group. The distribution ofoperations allows complex operations to be handled by a plurality ofservers, rather than by a single computer in a standard system.

If the cluster of size is small (e.g., 5) and penta-groups are used,there will be buckets that share the same group. As the cluster sizegrows, buckets are redistributed such that no two groups are identical.

Various embodiments of the storage system may allow taking a snapshot tosave a data set at a particular point in time. The snapshot maycomprise, for example, differential information with respect to aprevious snapshot or an image. The storage system may allow snapshots ata file system level such as, for example, a bucket level. When thesnapshot data is managed at the bucket level, the snapshot workload maybe shared across all buckets. Therefore, each bucket manages all thesnapshots for all its managed objects such as, for example, directories,inodes, file data ranges, etc. A processor, such as, for example, aprocessor of the host 201 or on a processor of the network adaptor 218,or any other processor that may be appropriate as shown on FIGS. 1 and2, may be used for managing the snapshots. For example, the snapshotmanagement may be performed by the leader of a bucket. The snapshotmanagement may be visible to a user via, for example, a user interface.

FIG. 4 illustrates a user interface for an example host of a storagesystem in accordance with an example implementation of this disclosure.Referring to FIG. 4, there is shown a host 400 communicatively coupled,either wired or wirelessly, to the user interface 410. The host 400 maybe similar to the host 201. The host 400 may also have, for example, aninput/output (I/O) interface 402 that allows communication with the userinterface 410. The I/O interface 402 may also comprise one or more inputdevices such as, for example, mouse, trackball, keyboard, buttons, touchpanel, etc. that allows a user to enter information. The I/O interface402 may also have one or more output devices such as, for example,lights/LEDs, speaker, display, etc. that allows a user to see or hearvarious outputs. Accordingly, the host 400 may have a display 404. Thehost 400 may also have, for example, a transceiver 406 that may besuitable for communicating with another electronic device via one ormore wired protocols and/or one or more wireless protocols. Wiredprotocols may be, for example, USB, Firewire, SCSI, etc. Wirelessprotocols may be, for example, a cellular protocol, WiFi, Bluetooth, NFC(Near Field Communication), etc.

Accordingly, the I/O stack 210, the TCP/IP stack 220, and/or the networkadaptor 218 may be thought of as being a part of the I/O interface 402.The I/O interface 402 may be a logical grouping of input/output devicesand applicable software.

Various embodiments may also communicate with the user interface 410,which may be similar to the I/O interface 402.

Depending on the application software, the user interface 410 and/or theI/O interface 402 may be used to access information, status, etc. forany level of the storage system. For example, the access may be to aspecific node, a file system, the entire storage system, etc.

A snapshot may be taken on demand by a user, periodically, or at certainset times. When a snapshot is allowed to be taken may be a design and/orimplementation decision. The user may determine when the snapshots aretaken. Various embodiments may take an initial snapshot of a data setfor a bucket, and subsequent snapshots may copy the changed data. Asystem of pointers may be used to reference the initial snapshot andsubsequent snapshots.

Each inodeid for a file (or directoryid for a directory, etc.) may alsohave a snapshot index (or snapshotID). There may also be a constantspecial snapshot index that never changes. This special snapshot indexis the current view (or the latest snapshot) of the file system, and maybe referred to as “defaultSnapID.” This special snapshot index mayconstantly store the differential information as the file system ischanged, and a snapshot will keep that information at a particular pointin time. When the snapshot is taken, the snapshot information will besaved at a new snapshot index and the defaultSnapID will gather newinformation from that point on.

A full inodeId may comprise, for example, the basic inodeId+snapshotID.The registry may keep track for each object it manages the “current”instance for that object by using inodeId+defaultSnapID to locate thecurrent snapshot.

As snapshots may be taken multiple times, there may be a tree ofsnapshots that is managed by the configuration service of the cluster ofnodes for the storage system. The configuration service of the clustermay keep track of, for example, what nodes are available, and whatcurrent buckets are on the storage system. Accordingly, all buckets knowthe relationships among snapshots. A snapshot may be written to since itis a “clone,” and a snapshot may be taken of the altered snapshot. Buteven if the snapshot configuration has just a single line going forwardto the next snapshot, a knowledge of how the snapshots are ordered maybe needed to view a specific snapshot.

When writing over a file after a snapshot, the file has an extent withthe most up-to-date snapshotID, that extent may be altered. If an extentwith the latest snapshot index (snapshotID) cannot be retrieved from theregistry, the latest snapshotID may be fetched from the registry byfetching the special entry that asks for the latest extent for thisextentId, which the registry keeps. Another extent may then be createdwith the right extentId and the most up-to-date snapshotID. This latestextent now has a pointer to the previous extent that is relevant and astandard write may be performed. A previous extent is relevant if it hasthe snapshotID of the extent that was received from the registry.

All the changes that are made in a context of a snapshot may be kept inan OnDiskHash (ODH). That ODH can be used in case a snapshot is deleted.For example, all the entries of that ODH can be reviewed. Changes thatoccurred in a deleted snapshot may be pushed to later snapshots, and allother changes may be discarded. As an example, if there is filemodification information for a file that does not exist in the nextsnapshot, there is no need to keep that information if that snapshot isdeleted.

The ODH may contain all modifications that happened in the last snapshotin an un-ordered way. So even if an extent was modified few times, theremay only be one entry. If a file was created, but then deleted beforeanother snapshot is taken, the ODH will contain no reference to thatfile.

Users of the storage system may use a snapshot for a variety ofpurposes. For example, a user may use a snapshot to backup timeconsistent replicas to a different storage system (physical backup).This may allow users to create recurring time consistent replicas inorder to archive systems (logical backup). This may also allow users toderive multiple images by taking a replica of a single gold image andchange it (image management). Users may also use a snapshot to developand test a system under multiple scenarios using multiple replicas ofthe input data.

One embodiment of the disclosure may allow a user to take a snapshot ofan existing file system. For example, the snapshot may be at a bucketlevel, or some other level that may be at a higher or a lower level thanthe bucket level. The user(s) may be allowed to access the snapshotunder a dedicated directory name. The snapshot may also be madewritable. A snapshot of a snapshot may also be taken. This may beuseful, for example, for a writable snapshot that has some portion of itchanged, or for a read-only snapshot before it is made writable. Variousembodiments may also allow a snapshot to be deleted. A snapshot may alsoallow the source file system to be restored if the interveningsnapshots, if applicable, are available.

For example, if a snapshot of a file system is taken at time X, then thesnapshot may be used to restore the file system to its state at time X.If there are other snapshots from time X to time X+nP, where n may besome integer number of a time period P, then those snapshots may be usedto restore the file system to any time from time X to time X+nP. Theremay be some upper limit to the number of snapshots that may be taken.For example, some embodiments may allow an upper limit of 4,096snapshots per file system, where the file system may be, for example, abucket. Various embodiments may allow different numbers of snapshots.

Snapshots may be taken on the whole file system or some subset of thefile system. File system snapshots may be presented to the user as, forexample, a directory included in a snapshot directory under the rootfile system. The name of the directory may be specified in a commandthat creates a snapshot. An attempt to move a file or a directory in orout a snapshot directory may not be supported in some embodiments. Theresult of trying to move a file or a directory may be, for example, thesame as a system command to copy/move between file systems. A snapshotcan be created, and various embodiments may allow a snapshot to be madewritable, to be deleted, and/or to be updated to contain the currentcontent of the file system or another snapshot.

Accordingly, each snapshot may designated to work with a file system,such as, for example, a bucket, and the snapshotID that is a string,where the snapshotID is unique per file system. This may allow different“nightly backup” snapshots on many file systems or files.

FIG. 5A illustrates an example flow diagram of taking a snapshot inaccordance with an example implementation of this disclosure. Referringto FIG. 5A, there is shown the flow diagram 500 with blocks 502 to 506.At block 502, a snapshot may be started. The snapshot may be, forexample, on demand, periodically, or other times. For example, asnapshot may be made half-way through a business day, and then at theend of the business day.

At block 504, a new entry may be made for the snapshot. Snapshots may bemaintained in the file system configuration by using, for example,tables. Accordingly, the latest entry may be, for example, in a snapshottable and a file system (FS) view table. The entry in the snapshot tablemay have several fields that are filled. The fields may be, for example,for a snapshotID, a pointer to its parent, the left child, and the rightchild. There may also be, for example, a file system id that may beuseful for debugging. The file system view table may be, for example,for the user, and each entry may comprise a field for a name and a fieldfor a pointer to the snapshotID. The name may follow a name format setup by the user such that the name may be meaningful to the user.Accordingly, the new entry in the file system view table may point tothe new entry in the snapshot table.

At block 506, the entries for the latest snapshot may be populated withrelevant information. The registry may also be updated with objectsrelated to the latest snapshot, where a registry key for each object maybe calculated on (object key, snapshotID). Accordingly, differentversions of objects may be stored with different registry keys.

A source snapshot field may be generated upon the snapshot creation, andthis field may indicate either a snapshot or a “latest copy.” If lateron the source snapshot is deleted, this field becomes invalid.

A snapshot may be designated as being writable using, for example, aBoolean field. File/directory name may be designated for accessing thesnapshots, and a snapshot may be tagged with a time stamp that indicateswhen it was created.

In general, snapshots may be uniquely identified by a (file_system,name, type, file path) tuple. The type may indicate, for example,whether the snapshot is for the file system, a directory, file, or anyother attribute that may be desired.

Accordingly, when taking (or creating) a snapshot, a new entry may becreated in the snapshot table, and the existing FS view table may bepointed to it. New FS objects may be created with the new snapshotID.The existing FS view may be pointed to the latest snapshot via itssnapshotID. Regarding references in the registry, the registry key maybe calculated on the parameters (object key, snapshotID). Thus,different versions of objects can be stored with different registrykeys.

FIG. 5B illustrates an example flow diagram of changes to a file systemin accordance with an example implementation of this disclosure.Referring to FIG. 5B, there is shown the flow diagram 520 with blocks522 to 524. At block 522, a change in a file system may take many forms.For example, there may be a write, a file or directory deletion, filetruncation, etc.

At block 524, the various changes may be handled. Regarding extent copyon write, a new extent may be created with the current snapshotID. Inplace of the block ids inherited from the previous snapshot, there maybe a reference to the snapshot index (snapshotID) of the extent thatowns them. If the snapshot context is the live file system, then thelatest extent pointer is updated. If there is a pointer in the placewhere an extent should be created, the pointer may be converted to afully-fledged extent.

Extent lookup comes with a snapshot context that includes thesnapshotID. The extent is searched with this snapshotID. If it is notfound, the bucket will traverse the snapshot tree upwards and search forthe extent using every snapshotID. All these searches will be in memory,unless there are hash collisions. Once an extent is found, if the soughtdata blocks are owned by ancestor extents, those blocks are retrieved inparallel, and then the data blocks can be read (or the block infodescriptors can be sent to the frontend).

When attributes of an inode are changed, the inode will be copied justlike an extent.

New dirents, or directory entries, will be created with snapshotID, justlike other objects. When deleting a dirent, the bucket may denote in thesnapshot that the dirent does not exist, so a “tombstone” may becreated. A tombstone may be a dirent that signifies that it has beendeleted. If this is not done, the original dirent will be found.

When truncating a file, the file system, for example, the bucket, scansfor the extents and leaves the tombstones where there are extents thatare supposed to be removed. There may not be a need for tombstones forsparse files with no extent, or past the end of the file.

When deleting a file, a tombstone may be left instead of the inode.Tombstones are not left for the removed extent. The tombstone is neededto facilitate merge on delete. When merging extents of a deletedsnapshot, it is useful to know if the extents exist in the childsnapshot. If the file is deleted with no tombstone there won't be a wayto know.

FIG. 6 illustrates an example flow diagram of deletion of a snapshot inaccordance with an example implementation of this disclosure. Referringto FIG. 6, there is shown the flow diagram 600 with blocks 602 to 606.At block 602, a command may be received to delete a particular snapshot.The command may be received via, for example, a user interface used formanaging the storage system, the file system, etc. The user interfacemay be a part of a node in the storage system, such as, for example, theI/O interface 402, or the user interface 410.

At block 604, as a snapshot is deleted, the file system, for example,the bucket, may enumerate the objects that belong to the snapshot andremove them. To do this, the bucket may have to scan up the tree to findthe first snapshot of the tree (the “−1 version”).

If there is no instance of the inode in the next snapshot, the inodeneeds to be moved to that snapshot. Otherwise the snapshot can beremoved. If the inode in the next snapshot is a tombstone, and there isno −1 version, they can both be removed. The process may also not searchfor the −1 version, and leave the tombstone.

Dirents may be treated the same as inodes for snapshot deletion.

If the extent is not present in the next snapshot, a pointer is createdin the next snapshot that points to the extent in the deleted snapshot.This may allow the original extent to still reference the blocks, whilethe registry is still able to reference the extent. The pointer is inthe object index of the next snapshot.

If an extent exists in the next snapshot, any blocks that are notreferenced from that extent are freed. If all blocks are freed, theextent itself can be removed.

At block 606, when deleting a snapshot that contains a pointer, thepointer is moved to the next snapshot if there is not an extent in thenext snapshot. If there is an extent in the next snapshot, the pointedextent should be examined as if its snapshot was just deleted.

FIG. 7 illustrates an example flow diagram of deletion of a snapshot inaccordance with an example implementation of this disclosure. Referringto FIG. 7, there is shown the flow diagram 700 with blocks 702 to 706.At block 702, objects may need to enumerated. Various embodiments mayuse different methods for enumerating, or identifying, FS objects thatbelong to a snapshot. For example, various embodiments may consider twooptions. At block 704, option 1 may be to use an on-disk hash table thatcan be reused in other places in the system. Or at block 706, option 2may be to maintain a double linked list between the objects. Option 2may cause more updates on creations and removals. While option 1 maywaste more capacity, since option 1 reuses an existing structure theremight be less code. While only one of option 1 or option 2 may be usedin an embodiment, both options are disclosed.

One snapshot (clone) is special—this is the live file system. The mostactivity can be expected to be on it, accordingly access to its objectsmay be made faster. This may be done by maintaining in the registry apointer to the last extent (or dirent or inode), and update it when anewer version is created in a snapshot.

When a file level snapshot is created, the inode is duplicated, and anew dirent is created as an entry point. File level snapshots may bemanaged just like file system level snapshots. That is, there is asnapshot tree in the configuration, and the extents are handled the sameway.

For an interface for accessing file system level snapshots, there may besome options. For example, one option may have the new snapshot exportedas a file system and mount the file system to be accessed. Anotheroption may be to have a snapshots directory under the root directory,with the snapshot root of every snapshot in there.

With regard to a rebuild, data blocks are always directly referenced byone extent—the extent that owned them when they were written. Theirbackpointers point to that extent. Therefore rebuild works as usual.

For restoring from a snapshot, this may consist of pointing the FS viewto another snapshot, and deleting the previous one. There may bevalidation to ensure that there are no open files on the view, and thenthe configuration may be changed.

In general, snapshots may be uniquely identified by the (file_system,name, type, file path) tuple. While there may be various operations fora snapshot, some commands may be, for example, Create Snapshot, UpdateSnapshot Parameters, Snapshot Copy, Restore From Snapshot, DeleteSnapshot, and List Snapshots.

Update Snapshot Parameters command may, for example, allow certainparameters to be updated. For example, whether the name may be changed.

Snapshot Copy may provide an instant copy of a snapshot or the currentcontent into an existing snapshot.

Restore From Snapshot may be used to restore the current content of afile system or file by the content of a snapshot.

List Snapshots may be used to list all snapshots that match the inputparameters. If no parameters are given, then all snapshots in the systemmay be listed.

Accordingly, it can be seen that an implementation of the disclosure maybe a method for using a distributed storage system that comprises takinga snapshot of a file system distributed across multiple storage nodes,maintaining the snapshot for the file system using a snapshot table anda file system views table, and providing a snapshot index (snapshotID)for accessing the snapshot, where a constant snapshot index is providedfor a current view of the file system. The snapshot may be writeable.

The file system may be distributed across five nodes. The snapshot tablemay comprise one or more entries, and each of the one or more entriesmay have a snapshot index field, a pointer to its parent, a pointer to aleft child, and a pointer to a right child. Furthermore, the snapshottable may comprise a file system identification.

The file system views table may comprise one or more entries, and eachof the one or more entries has a name field and a pointer to acorresponding snapshot. The method may also comprise enumerating filesystem objects that belong to the snapshot, where the enumeration may bedone via an on-disk hash or the enumeration may be done withdouble-linked lists for the file system objects.

Another implementation of the disclosure may be a distributed storagesystem that comprises a file system distributed across multiple storagenodes. The file system may comprise a processor, where the processor isconfigured to: take a snapshot of the file system, maintain the snapshotfor the file system using a snapshot table and a file system viewstable, and provide a snapshot index for accessing the snapshot, where aconstant snapshot index is provided for a current view of the filesystem.

The file system may be distributed across five storage nodes. Theprocessor may be configured to populate one or more entries for thesnapshot table, wherein each of the one or more entries has a snapshotindex field, a pointer to its parent, a pointer to a left child, and apointer to a right child. The processor may be configured to populate afile system identification field in the snapshot table and to populateone or more entries for the file system views table, wherein each of theone or more entries has a name field and a pointer to a correspondingsnapshot.

Still another implementation of the disclosure may be a machine-readablestorage having stored thereon, a computer program having at least onecode section for taking snapshots in a storage system, the at least onecode section comprising machine executable instructions for causing themachine to perform steps comprising taking a snapshot of a file systemdistributed across multiple storage nodes, maintaining the snapshot forthe file system using a snapshot table and a file system views table,and providing a snapshot index for accessing the snapshot, where aconstant snapshot index is provided for a current view of the filesystem.

The file system may be distributed across five nodes. The snapshot tablemay comprise one or more entries, and each of the one or more entriesmay have a snapshot index field, a pointer to its parent, a pointer to aleft child, and a pointer to a right child. The snapshot table maycomprise a file system identification. The file system views table maycomprise one or more entries, and each of the one or more entries mayhave a name field and a pointer to a corresponding snapshot.

While the present method and/or system has been described with referenceto certain implementations, it will be understood by those skilled inthe art that various changes may be made and equivalents may besubstituted without departing from the scope of the present methodand/or system. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the presentdisclosure without departing from its scope. Therefore, it is intendedthat the present method and/or system not be limited to the particularimplementations disclosed, but that the present method and/or systemwill include all implementations falling within the scope of theappended claims.

As utilized herein the terms “circuits” and “circuitry” refer tophysical electronic components (i.e. hardware) and any software and/orfirmware (“code”) which may configure the hardware, be executed by thehardware, and or otherwise be associated with the hardware. As usedherein, for example, a particular processor and memory may comprisefirst “circuitry” when executing a first one or more lines of code andmay comprise second “circuitry” when executing a second one or morelines of code. As utilized herein, “and/or” means any one or more of theitems in the list joined by “and/or.” As an example, “x and/or y” meansany element of the three-element set {(x), (y), (x, y)}. In other words,“x and/or y” means “one or both of x and y”. As another example, “x, y,and/or z” means any element of the seven-element set {(x), (y), (z), (x,y), (x, z), (y, z), (x, y, z)}. In other words, “x, y and/or z” means“one or more of x, y and z”. As utilized herein, the term “exemplary”means serving as a non-limiting example, instance, or illustration. Asutilized herein, the terms “e.g.” and “for example” set off lists of oneor more non-limiting examples, instances, or illustrations. As utilizedherein, circuitry is “operable” to perform a function whenever thecircuitry comprises the necessary hardware and code (if any isnecessary) to perform the function, regardless of whether performance ofthe function is disabled or not enabled (e.g., by a user-configurablesetting, factory trim, etc.).

What are claimed:
 1. A method for a distributed storage system,comprising: taking a snapshot of a file system distributed acrossmultiple storage nodes; maintaining the snapshot for the file systemusing a snapshot table and a file system views table; and providing asnapshot index for accessing the snapshot, wherein a constant snapshotindex is provided for a current view of the file system.
 2. The methodof claim 1, wherein the file system is distributed across five nodes. 3.The method of claim 1, wherein the snapshot table comprises one or moreentries, and each of the one or more entries has a snapshot index field,a pointer to its parent, a pointer to a left child, and a pointer to aright child.
 4. The method of claim 3, wherein the snapshot tablecomprises a file system identification.
 5. The method of claim 1,wherein the file system views table comprises one or more entries, andeach of the one or more entries has a name field and a pointer to acorresponding snapshot.
 6. The method of claim 1, wherein the snapshotis writeable.
 7. The method of claim 1, comprising enumerating filesystem objects that belong to the snapshot.
 8. The method of claim 7,wherein the enumeration is done via an on-disk hash.
 9. The method ofclaim 7, wherein the enumeration is done with double-linked lists forthe file system objects.
 10. A distributed storage system comprising: afile system distributed across multiple storage nodes; and a processor,wherein the processor is configured to: take a snapshot of the filesystem; maintain the snapshot for the file system using a snapshot tableand a file system views table; and provide a snapshot index foraccessing the snapshot, wherein a constant snapshot index is providedfor a current view of the file system.
 11. The system of claim 10,wherein the file system is distributed across five storage nodes. 12.The system of claim 10, wherein the processor is configured to populateone or more entries for the snapshot table, wherein each of the one ormore entries has an identification (ID) field, a pointer to its parent,a pointer to a left child, and a pointer to a right child.
 13. Thesystem of claim 12, wherein the processor is configured to populate afile system identification field in the snapshot table.
 14. The systemof claim 10, wherein the processor is configured to populate one or moreentries for the file system views table, wherein each of the one or moreentries has a name field and a pointer to a corresponding snapshot. 15.A machine-readable storage having stored thereon, a computer programhaving at least one code section for taking snapshots in a storagesystem, the at least one code section comprising machine executableinstructions for causing the machine to perform steps comprising: takinga snapshot of a file system distributed across multiple storage nodes;maintaining the snapshot for the file system using a snapshot table anda file system views table; and providing a snapshot index for accessingthe snapshot, wherein a constant snapshot index is provided for acurrent view of the file system.
 16. The machine-readable storage ofclaim 15, wherein the file system is distributed across five nodes. 17.The machine-readable storage of claim 15, wherein the snapshot tablecomprises one or more entries, and each of the one or more entries has asnapshot index field, a pointer to its parent, a pointer to a leftchild, and a pointer to a right child.
 18. The machine-readable storageof claim 17, wherein the snapshot table comprises a file systemidentification.
 19. The machine-readable storage of claim 15, whereinthe file system views table comprises one or more entries, and each ofthe one or more entries has a name field and a pointer to acorresponding snapshot.
 20. The machine-readable storage of claim 15,wherein the snapshot is writeable.